Defense lawyer Michael Arif unsuccessfully argued that the letter, written to convicted sniper John Muhammad's 17-year-old niece in the summer of 2002, should be allowed because it showed Malvo's state of mind before the sniper spree that killed 10 people in the Washington area.
Circuit Judge Jane Marum Roush rejected the request Wednesday, saying the letter was hearsay.
Malvo wrote the letter to LaToria Williams during a visit with Muhammad to the older man's family in Baton Rouge, La. Muhammad's relatives said they saw it as a cry for help.
"When he left, I felt he was crying out for help," testified Shelia Tezano, of Baton Rouge, sister of Carol Williams, Muhammad's first wife.
Defense lawyer Craig Cooley had referred to the letter during his opening statement Nov. 13, saying, "Lee kind of got a crush on one of John's nieces and wrote out a letter that he was despondent ... . He said that his mother had abandoned him, and I have a father who will kill me for a righteous society to prevail."
Carol Williams told jurors Tuesday during Malvo's capital murder trial that Muhammad had introduced Malvo as his son during the visit to Baton Rouge.
"Lee was the most well-mannered, respectful child," Carol Williams said. "Everyone just fell in love with him because he was so nice."
Carol Williams said she became concerned, though, when Malvo wrote the letter, "asking for help to get out of the situation that he was in." She said she and her sisters talked about what they could do, but that she thought Malvo and Muhammad had left Baton Rouge by then.
Malvo's lawyers are presenting an insanity defense, contending that Muhammad brainwashed Malvo and molded him into a killer to help him carry out the sniper attacks in and around the nation's capital during a three-week period in October 2002.
Malvo, 18, is being tried on charges of captial murder in the shooting death of FBI analyst Linda Franklin outside a store in Fairfax County.
During cross-examination Tuesday, prosecutor Robert F. Horan Jr. asked Carol Williams whether Malvo "ever expressed to you any fear of John Muhammad?"
She quietly replied, "No."
Williams' son with Muhammad also testified Tuesday, describing his father as "a manipulator" who took advantage of other people's weaknesses.
Lindbergh Williams, 21, stayed with Muhammad in Washington state for a summer 10 years ago under a custody arrangement. He said that Muhammad convinced him his mother had abused him.
He said it took him several months to trust his mother again after he was returned to her.
Lindbergh Williams also testified that he still loves his father.
Defense attorney Thomas Walsh said after court that Lindbergh Williams planned to visit Muhammad Wednesday in jail.
Muhammad was convicted last month of capital murder and a jury recommended the death sentence. Malvo also faces the death penalty if convicted.
The two are charged in or linked to the killing of 10 people and the wounding of six in the D.C. area, plus shootings in Washington state, Arizona, Georgia, Alabama and Louisiana. John Sinquefield, first assistant district attorney in East Baton Rouge Parish, La., said Tuesday that Muhammad and Malvo would likely be extradited in April or May to face charges there.